1. Field of the Invention
The present invention relates to a grinding tool of the type in which the abrasive is diamond known as the hardest abrasive.
2. Description of the Prior Art
The recent development of new materials is remarkable. Most noteworthy among them is a cermet of titanium nitride which is a new species of ceramics. It is a new material having characteristics combining the properties of ceramics and metals, such as high hardness and heat resistance characteristic of ceramics and toughness characteristic of metals. It is finding more uses as a material for high-performance cutting tools such as throwaway chips and reamers, in place of the conventional cemented carbides and old ceramics.
Heretofore, a grinding tool made of diamond superabrasive grains has been in use for truing and sharpening the cutting tools made of cermet. However, it has the disadvantage of being poor in grinding efficiency and life because of the difficulties encountered in grinding the hard cutting material.
In the case of titanium nitride cermet having high hardness and low thermal conductivity, the pressure and temperature at the grinding point are much higher than those which are experienced in the grinding of the conventional cemented carbide. This easily wears the superabrasive grains forming the cutting edge of the grinding tool, and in an extreme case, induces the crushing and releasing of the superabrasive grains. A grinding tool usually undergoes, before use, dressing to improve and keep its sharpness(free-cut performance). Dressing makes the cutting edge of the superabrasive grains project several tens of micron from the surface of the bond. When the amount of projection decreases to ten micron or less due to wear of the cutting edge, the grinding resistance rapidly increases due to friction between the work or chips thereof and the bond, and the temperature at the grinding point and its adjacent parts rises still more due to friction heat. This in turn causes fatal burn mark, crack, and chipping to the work, and also accelerates the attrition wear and breaking down of superabrasive grains, shortening the life of the expensive grinding tool to a great extent. In addition, at high temperatures, titanium nitride cermet has a tendency that the metal component thereof readily sticks to the surface of the grinding tool. This results in the surface of the grinding tool being loaded with the metal component and the grinding tool becoming dull very soon.
The above mentioned phenomena greatly reduces the sharpness and Life of the grinding tool and deteriorates the finishing performance of the grinding tool in a short time.
The following are the major countermeasures conventionally taken so far to cope with the problems mentioned above.
(1) Using superabrasive grains of comparatively small grain size (from 230 to 270 mesh, 64 .mu.m on the average).
Superabrasive grains of large grain size form a large flat area at the grain tips when they have worn out, which in turn increases grinding resistance, generates grinding heat, and causes edge chipping. Superabrasive grains of excessively small grain size, however, are easily released from the bond because they are not firmly retained thereto.
(2) Using an organic polymeric substance as the bond which is superior in self-lubrication to a metal-based bond and has a proper degree of resilience.
The metal-based bond generates a large amount of friction heat when it comes into direct contact with the work after the abrasive grains have worn out. In addition, it makes such a stiff contact with the work that it causes chipping to the cutting edge of the tool being ground.
(3) Incorporating the bond with a solid film-forming lubricant such as graphite, boron nitride of hexagonal system, tungsten disulfide, and molybdenum disulfide.
This is effective in reducing friction heat that generates between the bond and the work.
(4) Incorporating the bond with a fine powder of silver or copper.
This is effective in increasing the thermal conductivity of the bond and the dissipation of friction heat.
(5) Using as the bond a porous metal matrix impregnated with a liquid resin.
This type of bond has both the thermal conductivity characteristic of the metal bond and the resilience characteristic of the organic polymer bond.
(6) In corporating the bond of organic polymeric substance with 10 to 30% (by volume) of lead powder.
This is effective in preventing the grinding tool from loading with sticking chips and in improving the thermal conductivity of the bond.
The above mentioned countermeasures, however, have not proved themselves to be highly effective. It is still a great difficulty to efficiently and neatly grind hard cutting materials such as titanium nitride cermet. The efficiency of grinding the cutting tools of titanium nitride cermet is only one-several or one-several tenths of that of grinding the cutting tools of conventional cemented carbide.